The E.coli Repressor of Biotin Biosynthesis (BirA) is both an enzyme and a transcriptional repressor. The protein catalyzes synthesis of biotinyl-5'-AMP from the substrates biotin and ATP. BirA also catalyzes transfer of the biotin moiety from the adenylate to a lysine residue of the BCCP of the acetyl CoA carboxylase and binds sequence specifically to the forty base pair biotin operator sequence to repress transcription of the biotin biosynthetic genes. The adenylated form of biotin, bio-5'-AMP, is the activated intermediate in the enzymatic transfer of biotin to BCCP as well as the positive allosteric effector for binding of BirA to the biotin operator sequence. The energetics of interaction of the two ligands, biotin and bio-5'-AMP, with BirA have been determined using kinetic methods. Results of these studies indicate that the DG for binding of biotin to the protein is approximately -10kcal/mole,while that for binding of the adenylate is approximately -14 kcal/mole. Van't Hoff analysis of the binding reactions indicate that both reactions have large favorable enthalpies: -13 kcal/mole for biotin and -12 kcal/mole for bio-5'-AMP and there is no heat capacity change. The direct calorimetric measurements of the interaction of the two ligands with BirA is to compare the calorimetric enthalpies with the values of the van't Hoff analyses and to investigate the apparently negligible heat capacity change. This is of particular interest in light of independent evidence supporting the occurrence of conformational changes in the protein upon binding of the ligands.